The conventional control device for a variable displacement hydraulic pump includes a variable displacement hydraulic pump 2 (hereinafter, referred to a hydraulic pump 2) driven by an engine 1, and a pilot pump 3 (for example, refer to Japanese Patent Application Publication No. 5-53948). The hydraulic pump 2 has a swash plate angle, which is controlled by a servo piston 4, and is connected to a servo valve 5 for controlling the operation pressure of the servo piston 4. The servo valve 5 is controlled by a neutral control valve 6 (hereinafter referred to as a NC valve 6), a cut-off valve 7 (hereinafter referred to as a CO valve 7), and a variable torque control valve 8 (hereinafter, referred to as a torque control valve 8), which are connected in series.
A conduit 12a, branching from a discharge conduit 12 of the hydraulic pump 2, connects an actuating portion 7e of the CO valve 7 and an actuating portion 8d of the torque control valve 8 via branch conduits 12d and 12c, respectively. A conduit 13a branching from a discharge conduit 13 of the pilot pump 3 connects to a conduit 13b. An engine speed sensor 1a for detecting the engine speed of the engine 1 is connected to a control device 10 via an electric circuit 9. The control device 10 is connected to an operating portion 8c of the torque control valve 8 via an electric circuit 11.
A direction switching valve 16, connected to the discharge conduit 12, is connected to a hydraulic cylinder 20 via conduits 21a and 21b, and is connected to a jet sensor (a pressure detecting portion) 17 via a conduit 18. The jet sensor 17 is connected to a drain conduit 19. The discharge conduit 13 is connected to a pressure controller 14, which is equipped with an operating lever 15. The pressure controller 14 is connected to opposite actuating portions of the direction switching valve 16 via conduits 14a and 14b. 12b is a relief valve.
Now, the operation will be explained. The value of the pressure, detected at the jet sensor 17, is inputted via conduit 23 into the operating portion 6d at one end of the NC valve 6, and the value of the pressure, detected at the drain conduit 19 at the downstream side of the jet sensor 17, is inputted via conduit 22 into an operating portion 6c at the other end. The NC valve 6 is switched by the pressure difference of sites before and after the jet sensor 17. The NC valve 6 is moved to its position 6b when the direction switching valve 16 is moved to either of its end operating positions. By placing the direction switching valve 16 at the center valve position as illustrated in the drawing, the entire discharge of the hydraulic pump 2 is drained through the jet sensor 17 and the drain conduit 19 into the sump tank. Therefore, the pressure difference before and after the jet sensor 17 is greater and the NC valve 6 is at its position 6a in the drawing.
At this time, the engine speed signal, from the engine speed sensor 1a, is inputted into the control device 10, and in response to the engine speed signal, a command signal of the control device 10 is inputted into an actuating portion 8c of the torque control valve 8. The discharge pressure of the hydraulic pump 2 is also inputted into the actuating portion 8d of the torque control valve 8. When the discharge pressure of the hydraulic pump 2 is low, relative to the command signal of the engine speed, the torque control valve 8 is at its position 8a as illustrated in the drawing. When the CO valve 7 is at its position 7a and the NC valve 6 is at its position 6a, the pilot pressure from the conduit 13b is inputted into the actuating portion of the servo valve 5, therefore the servo valve 5 is switched to its position 5a. As a result, the servo piston 4 moves toward the left in the direction of an arrow Y, since the oil at the bottom side is drained and the oil from the conduit 13a flows into the head side, and the discharge of the pump 2 is increased.
Contrary to the above, when the discharge pressure of the hydraulic pump 2 is high, relative to the command signal of the engine speed, the torque control valve 8 is switched to its position 8b. As a result of this switching, the pilot pressure from the conduit 13b is not inputted into the actuating portion of the servo valve 5; therefore, the servo valve 5 is switched to its position 5b. As a result, the oil from the conduit 13a flows into the bottom side of the servo piston 4 and the oil at the head side is drained; therefore, the servo piston 4 moves to the right in the direction of the arrow Y, and decreases the discharge of the pump.
The force of a spring 7c is set at high, relative to the discharge pressure of the hydraulic pump 2, so that the CO valve 7 is normally at its position 7a. When the hydraulic pump 2 has the maximum pressure, the CO valve 7 is switched to its position 7b, so that the cut-off control of the maximum pressure is carried out. In response to the engine speed N and the discharge pressure P of the hydraulic pump 2, the torque control valve 8 controls so that the discharge Q (Q=q.multidot.N) of the hydraulic pump 2 is constant. The q is discharge per revolution (cc/rev). Accordingly, the absorption horsepower of the hydraulic pump 2 is controlled so as to be on a constant line of equal horsepower (P.multidot.Q=constant).
The above-described control device 10 of the hydraulic pump 2 conducts a control to reduce the discharge Q of the hydraulic pump 2 when the load during operation is increased and the discharge pressure P of the hydraulic circuit is risen. Specifically, explaining with reference to FIG. 2, the discharge Q moves on a line A (P.multidot.Q=constant). Therefore, the discharge of the hydraulic pump 2 begins to decline in response to the increased load before the engine speed is reduced, so that the speed of the cylinder 20 is reduced and there is a disadvantage of lacking firm responsiveness. Accordingly, when the load is suddenly increased during operation, it is necessary that the absorption horsepower of the hydraulic pump 2 is increased.
In addition, in order to improve the fuel efficiency of the engine 1, when the hydraulic pump 2 reaches the maximum pressure, the control device 10 conducts a cut-off control of the pressure. Explaining with reference to FIG. 4, the discharge pressure P is moved from a point C1 to a line C2 as a result of the cut-off control. When the discharge pressure P is moved to the line C2, matching and relief (relief flow Rq) are conducted at the point C3 with the relief valve property being set at the line R, and the control to minimize the swash plate angle of the hydraulic pump 2 is conducted. With the minimum swash plate angle, there is a disadvantage of lacking sufficient power.
As another control device for the hydraulic pump, a control is known in which a mechanism for delaying the operation of the regulator for controlling the hydraulic pump flow is included, and the delay time is increased or decreased according to the increase or decrease in the engine speed (for example, refer to Japanese Patent Application Publication No. 59-26796).
However, the above control device normally controls the hydraulic pump flow by increasing or decreasing the engine speed; therefore, there is a disadvantage of delaying the response of the swash plate angle of the hydraulic pump when the load is abruptly and repeatedly increased and decreased in a short time during operation.